1. Field of the invention
The present invention relates to an optical amplifying method for adjusting control parameters in an automatic constant gain control circuit to control gain of an optical amplifier, an optical amplifying apparatus and an optical amplified transmission system using the apparatus.
2. Related Art
A conventional optical amplifying apparatus 110, for example as shown in FIG. 19, includes an optical amplifier 111 connected to an optical transmission line 101. An optical signal inputted into the optical amplifier 111 and an optical signal outputted from the optical amplifier 111 and passed through an erbium-doped fiber (EDF) 112 are divided by optical couplers 113, 114 respectively, and then optical input power and optical output power thereof are detected by photo diodes (PD) respectively in the optical amplifying apparatus 110. The optical input power and the optical output power are converted to voltages corresponding to the optical powers in monitor circuits 117, 118, and the voltages are outputted to a control circuit 119, respectively. The control circuit 119 controls a pump laser diode 120 based on the voltages to implement an automatic constant gain control (AGC) in the optical amplifier 111, thus controlling the gain of the optical amplifier to be a prescribed value.
As one of the optical transmission system, there is a wavelength division multiplexing (WDM) system in which a plurality of optical signals of different wavelengths are division-multiplexed and simultaneously transmitted on a single fiber-optical transmission line. The optical signals are optically amplified through a plurality of optical amplifying apparatuses placed in the optical transmission line, and then transmitted to an optical transmission line of a single-mode fiber (SMF) or a dispersion-shifted fiber (DSF) in the WDM system.
In the optical amplifying apparatus implementing the AGC, the optical output power is controlled so as to maintain the gain to be constant, when the number of wavelengths in the WDM optical signal varies and the optical input power to the optical amplifier varies accordingly. However, when a transient characteristics of power control of the optical amplifier is not appropriate in the transitional period when the optical input power abruptly varies, the control of the optical output power corresponding to a variation of the optical input power of the optical amplifier is delayed. FIG. 20A shows the variation of the optical input power of the optical amplifier. FIG. 20B shows the result (i.e., delay). In this case, the optical output power per one wavelength channel largely varies (refer to FIG. 20C), thus deteriorating the transmission quality of the optical signal in the transitional period when the optical input power abruptly varies.
Japanese Patent Provisional Publication No. 2001-053682 discloses an optical amplifying apparatus providing a delay portion on the optical transmission line in which the variation of the number of the wavelengths is detected by the AGC circuit, and the transmission of the optical signal is delayed such that the time required for controlling the gain of the optical amplifier equals to the time required for inputting the optical signal to the optical amplifier, thus preventing the variation of the optical output power of the optical amplifier. More specifically, as shown in FIG. 21, the optical amplifying apparatus includes a delay portion 121 to delay the transmission of the optical signal on the optical transmission line between the optical coupler 113 at the input side of the optical amplifier 111 and the optical amplifier 111. In the optical amplifying apparatus, the time required for controlling the gain by the AGC circuit 119 is set to be the time required for the transmission of the optical signal, thus avoiding the variation of the optical output power as shown in FIG. 20B.
However, the above mentioned conventional optical amplifying apparatus cause a problem in which an additional space for locating an optical fiber in the delay portion is required, since the delay portion utilizes the optical fiber in order to delay optical signal on the transmission line, thus disturbing the realization of the down sizing of the apparatus.